Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Ink jet printing systems are generally of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up .into droplets at a fixed distance from the orifice. At this break-up point, the droplets are charged in accordance with digital data signals. These drops are then passed through an electrostatic field which adjusts the trajectory of each droplet. The droplets are either directed back to a gutter for recirculation or to a specific location on the substrate to create the desired character matrix.
In drop-on-demand systems, a droplet is expelled under pressure from a valve directly to a position on the substrate in accordance with the digital data signals. A droplet is not formed or expelled unless it is to be jetted to the substrate. Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream system.
In general, a drop-on-demand ink composition must meet certain rigid requirements to be useful in non-contact printing operations. These relate to viscosity, solubility, compatibility of components and penetration into the porous substrate. Further, the ink must penetrate quickly and be smear resistant, must be capable of passing through the print head valves without clogging, and must permit rapid cleanup of the machine components with minimum effort.
Moreover, for an ink composition to be useful for printing on porous substrates, such as paper and corrugated cardboard, it must adequately wet the substrate and penetrate the surface of the substrate. Presently-used ink for such applications conventionally contains a dye, a carrier, a liquid penetrant, a humectant, and other components. These humectants are generally glycols, polyglycols, or glycol ethers.
The penetrant and the humectant are volatile organic compounds which pose toxicity and flammability problems for the ink jet formulator. However, the use of volatile organic compounds is undesirable for an additional reason--they pose an environmental hazard. After the ink has been applied to the substrate, the volatile organic compounds evaporate, releasing organic compounds into the atmosphere. These volatile organic compounds adversely affect the environment in the atmosphere and are the subject of both federal and state governmental regulations.
Volatile organic compounds, also known as VOC, are defined as any compound of carbon that evaporates from a paint or coating film under specific test conditions (EPA Method 24 or ASTM Method D-3960). The amount of volatile organic compounds present in a given composition in grams per liter of coating can be determined by using the following formula: ##EQU1## where: W.sub.s is the weight of volatile compounds in grams;
W.sub.w is the weight of water in grams; PA1 W.sub.cs is the weight of exempt compounds in grams; PA1 V.sub.m is the volume of material in liters; PA1 V.sub.w is the volume of water in liters; and PA1 V.sub.cs is the volume of exempt compounds in liters.
In the above formula, exempt compounds are volatile organic compounds whose use is specifically sanctioned by regulatory agencies. Trichloroethane is an example of an exempt compound.
The maximum acceptable level of volatile organic compounds varies for different coating applications. For example, in California the maximum acceptable level of volatile organic compounds for porous printing applications is 300 grams/liter.
The problem of VOC content in ink compositions for non-porous printing applications has been addressed. For example, copending application Ser. No. 07/958,267, by the same inventors, discloses ink compositions comprising an aqueous resin solution and an organic pigment that have a VOC content of less than 500 grams per liter, and preferably less than 340 grams/liter. However, no such compositions have been found for drop-on-demand printing on porous substrates that comply with VOC regulations.
Moreover, as stated earlier, conventional inks for ink jet printing on porous substrates commonly contain glycol ethers, which act as humectants to prevent nozzle or valve clogging or tip drying in the print head, and to increase ink penetration into the substrate. These compounds, in addition to being volatile organic compounds, have also been found to be teratogenic and possibly carcinogenic.
From the foregoing, it is clear that a need exists for an aqueous drop-on-demand ink composition for printing on porous substrates that contains low levels of volatile organic compounds, and-is non-teratogenic and non-carcinogenic, and has good penetration and drying properties. The resulting composition would therefore, be environmentally friendly. In addition, the ink would pose no health hazard to people.